Exercise 4 Investigating Transition Metal Complexes 4 Introduction Colour is a well known property of the transition metals. The colour produced as parts of the visible spectrum are due to electron transitions between energy levels. The colour we see is due to absorption by electrons in the d orbitals therefore, if the atom has no d electrons or a full d orbital with 10 electrons, the compound will absorb outside the visible region, and appear white or colourless. Ligands Splitting the d orbitals In a transition metal atom the d orbitals all have the same energy level, however ligands split the d orbitals into two groups with a difference in energy of E. If the ligand of a transition metal is changed it alters the value for E and therefore the colour will also change. Factors affecting the E value include: The type and size of the ligand. The bond strength between the ligand and metal ion. The shape and co-ordination number of the complex. The oxidation state. UV-Visible Spectroscopy - Analysis of Colour A UV-visible spectrometer can be used to measure absorption of light in the ultra-violet and visible region. A typical spectrometer has two sources, usually a deuterium lamp to provide frequencies in the ultra-violet region and a tungsten lamp to provide the visible frequencies. To run a sample the instrument needs to compare the sample solution with a reference or blank solution. This reference solution is the solvent being used for the sample solution. The blank or reference is run first for a single beam instrument or in parallel with the samples for a double beam instrument. A diffraction grating or prism is used to separate the radiation into the different frequencies producing monochromatic radiation. The spectra plotted show the absorbance of the sample at particular wavelengths.
Method 1. Prepare a blank sample by filling a cuvette with the appropriate solvent and stopper with a lid. 2. The demonstrator will set up the spectrometer to scan the visible region from 350-900 nm. Run a blank and each sample as instructed. Print out the spectrum and note the wavelength for each of the absorbance peaks. 3. Investigate how the absorbance changes as the ligands are changed, record the changes in absorbance in the table provided. Exercise 4 - Investigating Transition Metal Complexes 2 Wavelengths of Different Coloured Visible Light Red 620-750 nm Orange 590-620 nm Yellow 570-590 nm Green 496-570 nm Blue 450-495 nm Violet 380-450 nm UV Region 190 nm 400 nm If a substance absorbs here... 450 nm 400 nm 800 nm - Visible Region Violet Blue Red 495 nm Visible Region Colour Wheel 620 nm Green Orange 570 nm Yellow 590 nm...it appears as this colour Beer-Lambert Law Experiment 1. You are provided with a solution of potassium permanganate of accurately known concentration (4.0 x 10-4 mol dm -3 ). 2. Use this solution to make up 50 cm 3 of each of the following potassium permanganate solutions using a 50 cm 3 burette to dispense your stock solution to the volumetric flasks. Concentration Stock solution in 50 cm 3 de-ionised water 3.2 x 10-4 mol dm -3 40 cm 3 2.4 x 10-4 mol dm -3 30 cm 3 3. Using the spectrophotometer measure the absorbance of each of your solutions in the visible region. The peak should occur at 540 nm. Also measure the absorbance of the unknown solution and record all data in the table provided. 4. Show that the Beer-Lambert Law is obeyed by plotting a graph of absorbance (y- axis) versus concentration (x-axis) using the graph paper provided. Draw the line of best fit through your points and the origin. Determine the concentration of the unknown solution from the graph. Questions 1. How do you know if the Beer-Lambert Law has been obeyed? 2. What is the concentration of the unknown solution? 1.6 x 10-4 mol dm -3 20 cm 3 0.8 x 10-4 mol dm -3 10 cm 3
Exercise 4 - Investigating Transition Metal Complexes 3 Materials Chemicals Transition Metal compounds selection of different colours eg copper sulphate, copper chloride, cobalt sulphate, vanadyl sulphate etc. Dilute ammonia solution 6M HCl Dilute sodium hydroxide De-ionised/distilled water Apparatus Disposable plastic cuvettes and stoppers Wash bottles x 4 100 cm 3 beakers 4 1 box pasteur pipettes and teats Tissues Spatulas Beer Lambert Law Chemicals Potassium permanganate solution 4.0 x 10-4 mol dm -3 (0.016 g in 250 cm 3 ) Unknown 25 cm 3 stock in 50 cm 3 De-ionised water Apparatus 5 x 50 cm 3 Volumetric Flasks Burette Funnel Disposable plastic cuvettes and stoppers Wash bottles x 4 1 box pasteur pipettes and teats Tissues Graph paper Instrument UV-visible Spectrometer (integral printer and paper) Laptop (optional) Printer (optional) Connection cables x 2 (optional) Set up for laptop and printer use: Connect UV-vis to laptop via left hand front USB port (Com 5) Connect printer to any USB port From spectrometer menu Select printer / auto print on / Computer USB / OK Open PVC program, set auto print to on or off depending on requirements.
Exercise 4 - Investigating Transition Metal Complexes 4 Results Changing Ligand [Cu(H 2 O) 6 ] 2+ Blue Octahedral 880 nm [Cu(NH 3 ) 4 (H 2 O) 2 ] 2+ Blue-Violet Octahedral 660 nm Changing Co-ordination Number [Cu(H 2 O) 6 ] 2+ Blue Octahedral 880 nm [CuCI 4 ] 2- Yellow Tetrahedral 880 nm and 380 nm Changing Oxidation State
Exercise 4 - Investigating Transition Metal Complexes 5 Beer Lambert Law 4.0 x 10-4 0.775 3.2 x 10-4 0.639 2.4 x 10-4 0.516 1.6 x 10-4 0.336 0.8 x 10-4 0.194 Unknown 0.395 Beer-Lambert Calibration Plot Potassium Permanganate Solution 0.900 0.800 0.700 0.600 0.500 0.400 0.300 0.200 0.100 0.000 0.0E+00 5.0E-05 1.0E-04 1.5E-04 2.0E-04 2.5E-04 3.0E-04 3.5E-04 4.0E-04 4.5E-04 Answers 1. The plot of absorbance versus concentration is a straight line graph. 2. From the graph the solution is 2.1 x 10-4 mol dm -3 the unknown solution prepared was 2.0 x 10-4 mol dm -3
Exercise 4 - Investigating Transition Metal Complexes 6 Student Work Sheet Changing Ligand [Cu(H 2 O) 6 ] 2+ [Cu(NH 3 ) 4 (H 2 O) 2 ] 2+ Changing Co-ordination Number [Cu(H 2 O) 6 ] 2+ [CuCI 4 ] 2- Changing Oxidation State
Exercise 4 - Investigating Transition Metal Complexes 7 Beer Lambert Law 4.0 x 10-4 3.2 x 10-4 2.4 x 10-4 1.6 x 10-4 0.8 x 10-4 Unknown Concentration of Unknown: